Cathode-ray tube



EXAMINER CROSS REFERENCE W M R 0R mp 9 v w. VW 2 a R w 6 Y B i 5 E M n m 1 his via. m .Y a P .m w .m H d .e m m F g v 6 4 9 1 m M Q0540 QkkbX \Q uovkkh Irranwsy Patented Oct. 15, 1946 CATHODE-RAY TUBE George W. Pratt, Lancaster, Pa., a'ssignor to Radio Corporation of America, a corporation of act XJOVJI Delaware Application January 30, 1945, Serial No. 575,273

I 10 Claims.

1 My invention relates to vacuum tubes in which adjacent regions of a glass or other vitreous Wall are electrically charged at widely different potentials, for example by conductive coatings spaced along the wall of the tube, and more particularly to cathode ray tubes with electrodes comprising conducting matings on the inner wall of the tube. Some types of cathode ray tu es 1n w ch an electron beam from an electron gun is directed lengthwise of the tube to a fluorescent screen on the opposite end of the tube have as part of the second anode of the, conventional electron gun a conductive coatin on the tube wall, and an auxiliary or post accelerator electrode, usually an annular coating of conducting material on the inner wall of the tube, between the second anode and the screen. The second anode and the accelerator electrode are separated by a gap or zone, which may be as much as an inch wide, of clean uncoated glass to permit a difference of potential of several thousand volts to be maintained between the second anode andthe accelerator electrode. Difliculty has been encountered due to spark discharges through this gap or zone along the surface of the glass between the adjacent edges of the second anode and the accelerator electrode particularly where the electrode coat ings contain enough graphite or similar carbonaceous material to produce the black and nonreflecting coatings enerally used in tubes of this type.

Attempts have been made to increase the breakdown potential of the gap between the two electrodes or coatings by applying at the adjacent edges of the electrodes highly conductive smooth coatings of striping material on the glass and in contact with the electrodes in the form of stripes of silver paste, which is a, suspension of powdered silver in an organic binder. i

The principal object of my'invention is to provide means by which in a tube of this type the breakdown potential between wall regions of widely different potentials, such as the adjacent edges of electrodes comprising conductive coatings, is reatly increased and arcing and spark discharges across the gap between such regions is minimized.

In accordance with my invention I apply to the wall of the bulb between the regions of different potential, such as the adiacent edges of the conductive coatings, a gas-free adherent .e of striping material comprising a fine powder, preferably fg ic oxide, Pesos, intimately mixed with an alkali metal silicate sugh a s so digm silicate, either as a coating which covers the surface of the glass wall or zone between the adjacent edges of the conductive coatings, or as a stripe along each of those edges. The striping material, whether applied as stripes or continuous coating, should overlap or otherwise be in electrical contact with all of the edge of the conductive coat e 8 2 ing. The preferred form of striping material is much like a oi t ng a pigment of ferric oxide, FezOs, in a vehicle of alkali metal silicate.

en a yer of this striping ma eria 1s epo-s1 e on the bulb surface from a, suspension of finely powdered F8203 in an aqueous solution of sodium silicate and baked out, for example, at abou o 0 1 about an hour, it produces'on TliWlfEfthe bulb an adherent, hard, smooth, substantially gas-free coating which appears to be almost electrically nonconductive and is dark red in color when the fine powder is ferric oxide, F6203. When this striping material is so used the breakdown potential across the gap between conductive regions on the bulb walls, such as the conductive coatings, is greatly increased. In conventional tubes having conductive coatings with the usual gap of about one inch between their adjacent edges, breakdown of the gap and arcing are apt to occur when the potential difference between the coatings is about/3000 to 4000 volts, which is frequently too low for best results in tubes of this kind. I have found that if the striping material of my invention is used, the potential difference may be increased to as much as 75,000 volts, and that the gap between thecoatings may be as small as about three-eighths of an inch without the occurrence of arcing or breakdown between the edges of the coatings.

My invention will better be understood in con-- nectlon with the accompanying drawing in which merely for purposes of illustration I have shown a, cathode ray tube of a commercial type having the second anode and the accelerator electrode in the form of annular conductive coatings on the interior of the bulb and in which Fig. 1 is a view in longitudinal section partly broken away of a cathode ray tube embodying my invention and having the striping material of my invention applied as an overlapping stripe along and coextensive with each of the edges of the conductive coatings, and Fig. 2 is a' fragmentary view of the middle portion of a similar tube with the striping material applied as a'coating which covers all of the interior surface of the bulb in the zone between the adiacent edges of the conductive coatings and overlaps the edges of the coatings to some extent.

The drawing illustrates a commercial cathode ray tube having a conventional glass or similar vitreous elongated envelope l with a flattened end on which there is a conventional fluorescent screen 2, and a neck in which there is schematically shown some electron discharge or beam-producing means, such as an electron gun of the type shown in U. S. patent to Holshouser, 2,348,- 216, May 9, 1944, an indirectly heated thermionic cathode, a first anode 3, and a second electrode 4. The beam deflection means, not shown, may be of the conventional electrostatic or electromagnetic type. The second electrode 4 of the electron gun is electrically connected to a conductive coating of graphite on the inner surface of the neck of the envelope adjacent the electrode 4 and preferably extendin a short distance into the flared portion of the envelope. The electrode 4 and the coating 5 constitute the second anode of the tube. The post accelerator electrode 6 is an annular electrode coaxial with the other electrodes and surrounding the path of the electron beam, and is preferably a black conductive coating of graphite on the wall of the envelope between the second anode and the fluorescent screen. A black conductive coating of graphite on the inner wall of cathode ray tube is well known in the art, and is disclosed in U. S. patent to Zworykin, 1,988,469, Jan. 22, 1935. Operating potentials are applied to the tube electrodes in the conventional way by a voltage source 1 shown diagrammatically. The electrodes cooperate to produce an electronic discharge lengthwise of the envelope in the form of an electron beam from the electron gun to the fluorescent screen.

The breakdown voltage over the clean uncoated glass surface forming the gap between two spaced conductors, such as the adjacent edges 8 of the coating 5 of the second anode and 9 of the accelerator electrode, may be increased by placing along the edge of each of the electrodes a stripe In of striping material coextensive with those edges as shown in Fig. 1 in which there is a zone ll of uncoated glass between the stripes or as shown in Fig. 2 by covering all the glass surface of the zone between the electrodes with a continuous coating 12 of striping material in electrical contact with the edges of both electrodes throughout the entire length of those edges. The striping material may to advantage be in contact with and electrically connected to the electrodes, for example, by overlapping the edges of the two electrodes preferably about 1 6 to /4 of an inch. All of the edge of the electrode should be covered by thestriping material, else sparking may occur from an uncovered part of the edge.

In accordance with my invention the striping material is deposited on the glass wall from a suspension of aflne pow der in an aqueous solutlon of an allghibitweam suchas gddgumfor potassium sil cate, and the deposit baked until the wa er s riven o and the deposit' converted into a hard adherent substantially gas-free coating of striping material composed of the fine powder dispersed through the dehydrated silicate. This coating is nonconductive or very slightly conductive. While I have obtained the best results with a mixture of sodium silicate and finely powdered ferric oxide iez Oa, wH1ch gives good results and is mined; other fine powders of insulating or semi-insulating material may be used. I have found that green chromic oxid can be substi, tuted in who e or ic oxide, u't

ductive materials. The nonconductive or very slightly conductive coatings made in accordance Wl y ven 1011 may e used as a continuous coating covering the entire area between the conductive coatings of electrodes, or may be used as narrow stripes on the glass overlapping the edges of the conductive coatings or electrodes, with a zone of clean glass between the stripes.

I have obtained good results with striping material prepared by dissolving 100 to 125 grams of 5 sodium silicate in 260 to 500 cubic centimeters of distilled water, which may be heated if desired, an a ng s owly and with constant stirring 500 grams of finely powdered ferric oxide, FezOs, stirring until all of the oxideddfifafid a'uhiform mixture free from lumps is obtained. The result is a kind of paint, with sodium silicate as the vehicle and ferric oxide as the pigment in which the pigment is the major constituent by weight. The weight of the FezOs is preferably from about two to five times the weight of the sodium silicate, but the proportions of the constituents can be varied considerably Without impairing the effectiveness of the coating. Other oxides of about the same electrical conductivity as ferric oxide, finely powdered metallic or finely powdered electrically nonconductive material may be substituted in whole or in part for the ferric oxide pigment. This paint may be applied to the glass envelope with a brush, or in any other convenient way, in as thin a coat as will produce the desired results. The application of the striping material to the glass surface to form a continuous coating between the edges of the electrodes may be somewhat facilitated by adding to the solution above described some binder, such as 5 grams U. S. P. gelatin, and a small amount, such as 1 cc., of ammonium-hydroxide.

The silica e coa mg ay e app ied to the bulb first and the conductive coating which forms the electrode may be applied to overlap the edge of the silicate coating, or the silicate coating may be applied after the conductive coating and overlap it at the edge. Either method is satisfactory, but the latter is somewhat more convenient.

I have also found that the breakdown or sparking potential between the edges of the electrodes may be greatly increased by making the narrow stripes at the edges of the electrodes of coating material consisting of a mixture of s q di ufn s ili; cate and fine particles of metal, such as silver,

gem or platinum which do not have noncone or fig resistance surface films, such as films of oxide. Such a coating is much more conductive then a coating containing ferric oxide. For example, the ohmic resistance of striping material consisting of sodiumasilicateaogtaining finely powdered sill' er and k 141,51 a ,g1a,.$,...=.1j.i e wide and long is fi filt 0.1 ohm. Apparently high ohmic resistance is not necessary in these narrow stripes of coating material, but is important and necessary where the coating material completely covers the glass surface between the electrodes.

I have found that conductive striping materials used as narrow stripes along the edges of the electrodes must be entirely free from carbon in any form, either as a constituent or as a residue from an organic binder, in order to be effective in increasing the potential difference at which breakdown or sparking occurs between the edges of the electrodes. The fact that the presence of carbon in any form in these stripes tends to prevent the increase in breakdown potential obtainable with stripes free from carbon is well established, although no acceptable explanation of that fact is at present available.

The way in which the striping material increases the breakdown voltage and prevents arcing across the gap between the edges of the electrodes is not fully understood. Heretofore the glass envelopes of cathode ray tubes of the kind in which my invention may be used to advantage were very carefully cleaned on the inside, particularly in the zone between the edges of the electrode coatings and, in addition, the tubes were treated or aged by maintaining for about two minutes a spark discharge between the edges of the electrodes, but nevertheless the breakdown potential could not in all cases be raised to the desired value and in no case to the value obtainable by using the ferric oxide mixture of my invention. It may be that the finely powdered material in the coating of striping material tends to distribute the electrostatic stress and make the potential gradient between the edges of the electrodes more uniform than it would otherwise be.

My invention is not limited to the particular type of tube shown by way of illustration, but is applicable to any electron discharge tube in which difierences of potential between adjacent regions of the tube walls are such that sparking or are discharges are liable to occur along the walls between such regions.

I claim:

1. A cathode ray tube comprising a highly evacuated elongated vitreous envelope enclosing electrodes for producing an electron discharge lengthwise of said envelope, two conductive coatings on the inner wall of said envelope and spaced along the length of said envelope, and an adherent substantially gas-free coating of finely powdered FezOa and sodium silicate on and covering said inner wall between the adjacent edges of said coatings and adjoining said edges.

2. A cathode ray tube comprising a highly evacuated vitreous envelope enclosing means for producing an electron discharge in said envelope, 2. second anode and an accelerating electrode consisting of annular conductive coatings on the tube wall and spaced along the tube coaxial with the path of said discharge and a coating consisting of finely powdered insulating or semi-insulating material dispersed through an alkali metal silicate on the tube wall between and electrically connected with both said anode and said accelerating electrode.

3. A cathode ray tube comprising a highly evacuated elongated vitreous envelope enclosing means for producing an electron discharge lengthwise of said envelope, an electrode on the inner surface of said envelope comprising conductive material, and a baked gas-free stable coating on the inner surface of said envelope and adjoining said electrode and consisting of the dehydrated residue of a suspension of finely powdered ferric oxide in a water solution of sodium silicate.

4. An electron discharge device comprising a highly evacuated vitreous envelope enclosing means for producing an electron discharge in said envelope, two cooperating electrodes comprising conductive coatings on and spaced along the inner wall of said envelope and surrounding the path of said discharge, and a coating of iron oxide and sodium silicate on said inner wall between the adjacent edges of said electrodes and in electrically conductive contact with all of said edges.

5. A vacuum device comprising an envelope of vitreous material enclosing means for producing an electron discharge in said envelope and having on its inner wall two spaced electrodes, and an adherent coating between and overlapping the adjacent edges of said electrodes and consisting essentially of a layer of baked dehydrated sodium silicate and finely divided ferric oxide uniformly dispersed throughout said layer, the weight of ferric oxide in said layer being from two t five times the weight of sodium silicate.

6. A vacuum device comprising an envelope of vitreous material enclosing electrodes for producing an electron discharge, means for producing on the inner walls of said envelope spaced adjacent regions at different potentials and an adherent coating on said wall between said regions consisting of a dried paint composed of a vehicle of an alkali metal silicate and a uniformly dispersed finely powdered substantially nonconductive metallic pigment constituting the major part by weight of the dried paint.

7. An electron discharge device comprising a glass envelope enclosing means for producing an electron discharge in said envelope, two cooperat ing electrodes surrounding the path of said discharge and comprising conductive coatings on and spaced along the inner wall of said envelope to provide between their adjacent edges a zone of clean uncoated glass wall, and a narrow stripe of finely powdered carbon-free electrically conductive material dispersed in a layer of dehydrated alkali metal silicate on said inner wall and extending along and. in contact with the edge of each of said electrodes and projecting into said zone less than half the width of said zone, said stripes being separated throughout their length by a clean uncoated zone on said inner wall.

8. An electron discharge device comprising a vitreous envelope enclosing means for producing an electron discharge in said envelope, two cooperating electrodes surrounding the path of said discharge and each comprising a conductive coating on the inner wall of said envelope and a narrow stripe of fine metal particles having an electrically conductive surface dispersed in a layer oi dehydrated alkali metal silicate on said inner wall and extending along and in contact with the edge of said conductive coating, said electrodes being spaced along said inner wall with the adjacent edges of the stripes of said electrodes separated by a clean uncoated zone on said inner wall.

9. An electron discharge device comprising a vitreous envelope enclosing means for producing an electron discharge in said envelope, two cooperating annular electrodes comprising conductive coatings on and spaced along the inner wall of said envelope and surrounding the path of said discharge, two narrow stripes of fine metal particles having high surface conductivity dispersed in a layer of dehydrated sodium silicate on said inner wall, said stripes being separated throughout their length by a clean uncoated zone on said inner wall and extending along and in contact with the edges of the respective electrodes.

10. An electron discharge device comprising a vitreous envelope enclosing means for producing an electron discharge in said envelope, two cooperating annular electrodes surrounding the path of said discharge and each comprising a conductive coating on the inner wall of said envelope and a narrow stripe of finely powdered silver dispersed in a layer of dehydated sodium silicate on said wall and extending along and in contact with one edge of said electrodes, said electrodes being spaced along said wall and separated by a clean uncoated zone on said inner wall coextensive with and between the adjacent edges of said stripes.

GEORGE W. PRATT. 

